allow custom install prefix

--- a/utils/build-presets.ini	2019-04-11 14:51:40.060259462 +0200
+++ b/utils/build-presets.ini	2019-04-11 15:16:17.471137969 +0200
@@ -752,7 +752,7 @@
 install-swiftpm
 install-xctest
 install-libicu
-install-prefix=/usr
+install-prefix=%(install_prefix)s
 install-libcxx
 install-sourcekit-lsp
 build-swift-static-stdlib